Although in some embodiments the invention may be applicable to waste materials other than radioactive waste, the present invention will be illustrated with particular reference to high level radioactive waste and spent nuclear fuel rod encapsulation.
High level radioactive waste usually is in the form of a solution of the waste. This solution can be calcined at 600.degree. C.-800.degree. C. temperature to provide the waste in powder form. The waste must be safely encapsulated in some system which is corrosion and heat resistent and will prevent the waste coming into contact with any ground water so that the possibility of contamination of ground water through leaching is avoided.
Alternatively, spent fuel rods could be simply disposed of in a safe containment system.
Various prior proposals have been published in relation to safe disposal of nuclear waste. One proposal is to immobilise the waste as a minor component of a synthetic rock matrix which is produced from mineral precursor components and the waste in powder form. The matrix is produced under conditions of high temperature and high pressure maintained for several hours. Suitable synthetic rock structures have been published by A. E. Ringwood et al, see for example the following:
Nature March 1979
European Patent Application No. 79301382.2
U.S. patent application Ser. No. 124953
Engineering techniques for production of synthetic rock incorporating the high level waste include hot isostatic pressing processes (see for example European Patent specification No. 0044381 assigned to ASEA) and hot uniaxial processes (see Australian patent application No. 18163/83 and equivalent U.S. Pat. No. 4,645,624 assigned to the assignees of the present application.
Another publication by ASEA concerns a suggested process in which a copper canister is filled with a mixture of unprocessed spent nuclear fuel and copper powder. Hot isostatic pressing is specified for the purpose of embedding the fuel in a dense copper matrix.
Yet another publication by ASEA (see U.S. Pat. No. 4,209,420 H. Larker) concerns a suggested process in which a ceramic canister (formed by hot isostatic pressing) is filled with a unprocessed spent nuclear fuel waste and a lid is placed in position. A gas-tight shroud of metal is placed around the canister. Hot isostatic pressing at very high pressure is specified for the purpose of joining the lid to the canister body.
Another proposal for nuclear fuel storage has been made in a paper entitled "Final Storage of Spent Nuclear Fuel" by Swedish Nuclear Fuel Supply Company (KBS Division). A massive copper canister having relatively thick walls receives spent nuclear fuel rods and copper powder fills the voids around the rods and within the canister. A lid is applied and the canister placed in a thin walled container. Evacuation of air from within the canister takes place and the outer container can then be sealed and passed to a hot isostatic pressing step operating typically at 500.degree. C. and the very high pressure of 150 MPa. The very high pressure is necessary to bond the lid to the container by sintering.
The proposals described in the previous three paragraphs are widely questioned as not viable on grounds including feasability and safety because of the very high pressures involved.
Yet a further proposal is to encapsulate the unprocessed spent nuclear fuel in a dense graphite nickel sulphide bonded matrix. It appears such a matrix can be formed at about 500.degree. C. but it is questionable whether the matrix would be considered safe for long-term storage because of an inherent brittle characteristic with the attendent risk of fracture and leaching of radioactive components by ground water. Furthermore, the long-term stability of the matrix in terms of its resistance to leaching does not appear to be established.
Accordingly, over a period of many years there have been extensive investigations at considerable cost in to many different kinds of systems for safely immobilising radioactive waste for disposal. With a view to providing a system having commercial attractivness with inherent acceptable safety, and the practicality to operate on a long-term basis, the present invention has been devised and provides new and useful alternatives to previous published proposals.